Is there an API to get iCloud Calendar data?

I am trying to create a web service that displays integrated calendars. The first source I am trying to add is iCloud Calendar. I have seen similar services such as Microsoft's Outlook app on iOS, and Todoist, both of which use a built-in web browser window to ask user to log in to iCloud and grant their permissions to access their calendar data. I think there should be some documentations on how to get iCloud Calendar data and possibly as CalDav, but failed to find any. Could any one give me some hints? Thanks!